Certain motor vehicles, heavy trucks that are powered by diesel engines for example, may be parked for extended periods of time. If the engine is allowed to keep running after a truck has been parked, the alternator should be effective to keep the truck's battery bank charged. If the engine is shut down, either intentionally by the driver, or automatically by a device such as an idle shutdown timer, while the circuits for electric devices remain on, the battery bank will begin to drain through those circuits. For example, an ignition switch will typically remain on after an idle shutdown timer has timed out, and certain circuits that are fed through the ignition switch and may have been left on may continue to draw current.
Because the battery bank must be able to provide sufficient current for cranking the engine at starting, the battery bank of a diesel-powered heavy truck typically comprises multiple storage batteries ganged together in parallel. If the battery bank is allowed to drain to the point where it cannot deliver the large cranking current necessary for starting the engine, the vehicle cannot be operated. Because a heavy truck may be parked for an extended period of time, it is therefore appropriate to guard against the possibility that its battery bank will be drained to a point that will not allow engine to be started or cranked due to failure to turn off all its electric circuits.
Consequently, it is known to place a disconnect switch in circuit between the battery bank and the rest of the vehicle electrical system for use in disconnecting the battery bank from the electrical system to prevent battery current draw that, if allowed to continue long enough, could drain the battery bank to the point of preventing the engine from being cranked and started.
Commonly owned U.S. Pat. No. 6,836,094 discloses an example of a battery disconnect circuit that has the advantage of a direct, switch-free connection of the battery bank to the cranking motor so that when the cranking motor solenoid is energized to crank the motor, cranking current flows directly from the battery bank to the cranking motor windings, and not through the disconnect switch.
Battery disconnect switches are typically electromechanical devices, not solid-state devices. The solid-state counterparts of certain electromechanical devices can often, but not always, provide significant advantages in performance and/or function and/or cost. Sometimes no comparable solid-state counterpart exists for an electromechanical device. For various reasons electromechanical devices continue to enjoy significant usage in the automotive industry, and it is believed reasonable to include battery disconnect switches in that category.
When an electromechanical battery disconnect switch fails, it is typically replaced, but sometimes it is simply bypassed for expediency. The inherent nature of such a switch seems to render it poorly suited for interfacing with vehicle electrical systems that have become largely electronic over the past several decades. Consequently, electronic monitoring of such switches for diagnostic, data collection, and other various purposes appears to have been largely ignored.